This invention relates to light-absorbing polymeric salts and to shaped polymeric structures, especially self-supporting films, coated therewith.
Photosensitive lithographic plates are readied for printing by exposing them to activating radiation through a negative, thereby rendering the exposed portions ink-receptive and the unexposed portions non-receptive in the case of a negative-acting plate, or vice versa in the case of a positive-acting plate. The higher the quality of printing desired, the more important it is to avoid exposing undesired portions of the plate to radiation. Such inadvertent exposure may occur when pinholes or other flaws exist in the negative, or when certain portions of the negative must be blocked out for use in a different printing operation. It is usually necessary to achieve an absorbance of 3 to 3.5 to satisfy the lithographic preparator. An early attempt to solve this problem involved "painting," with a radiation-obscuring composition, the portions to be blocked out, a technique that is usually effective but tedious and irreversible.
A more sophisticated way to cover areas of a negative is to apply so-called lithographers' tape, a product in which a strongly dyed or pigmented unplasticized polyvinyl chloride (UPVC) resin composition is extruded as a 50-micrometer film and thereafter coated on one side with a normally tacky and pressure-sensitive adhesive (PSA). Although this commercially popular tape is convenient to apply, effective, and removable, it is not completely uniform in appearance, hence may raise doubts about quality and thus is less aesthetically appealing than may be desired. Further, the colorants required do not exhibit a "sharp cut-off" and hence severely limit the amount of visible but lithographically inactive (non-actinic) light transmitted; this light is used by the lithographic preparator to verify that desired information has not been inadvertently covered. Additionally, the acetone or other solvent often used to clean the lithographic negative tends to dissolve the UPVC and may cause the color to leach from the film, contaminating adjacent areas and causing considerable annoyance to the printer.
Attempts have been made to incorporate dye in such solvent-resistant polymers as polyethylene terephthalate and polypropylene, but processing at the required high levels of dye has proved difficult and the higher extrusion temperatures tend to decompose or volatilize the dye. Those dyes that are stable at high temperatures are very limited in the number of colors available. If the dye is incorporated in the pressure-sensitive adhesive, a residual "ghost" color image often remains on the negative when the tape is removed.